In a mobile communication system, there has recently been suggested a scheme of a base station called femtocell (femtocell FAP) which covers a very small area compared to a conventional base station (wide area base station, macrocell). The femtocell is a small base station installed in a house, a small office and the like, for example. While a conventional cell of a mobile phone, that is, an area covered by a single wide area base station is about 1 to several kilometers in radius, the area covered by the femtocell is only about a few to tens of meters. The femtocell is connected to the mobile communication network via a public link (broadband link such as ADSL or the like) installed in each house. Since the public link is widely distributed as an access link, a user even outside the area of the macrocell can easily use mobile phone services (a telephone call, a message function, an SMS (Short Message Service), a WEB browsing function and the like) similar to those provided by the macrocell at low cost if the broadband link is available. In addition, there is an advantage for a carrier as well, that use of the femtocell and the broadband link by the user saves resources (bandwidth or the like) of an existing wide area base station, supposed to be consumed, allowing for an improvement in the area of mobile phones at low cost.
Therefore, it is expected that in 3 Generation (3G), 3.9 Generation (3.9G) and IMT-ADVANCED using a frequency band at 2 GHz or higher frequency, it becomes essential to combine the wide area base station installed outdoors and the femtocell installed indoors. In addition, since it is considered preferable to allow only registered terminals (mobile terminals) to use the femtocell, the femtocell can be configured to be occupied by only a limited number of users registered to use. Accordingly, the femtocell has an advantage to offer a faster data communication environment of better quality than the macrocell overcrowded with communications of multiple users, and thus it is expected to come into wide use in the future.
Because of the advantages of the femtocell described above, it is expected that a user capable of using a femtocell service wishes to connect to the femtocell when he enters the femtocell network from the macrocell network. As a method of handoff from the macrocell to the femtocell, it is considered to use a pilot beacon. Although it is desired that the femtocell uses a frequency different from that used by the macrocell in to avoid interference with the macrocell, it is preferred to transmit the pilot beacon at the same frequency as that of the macrocell to attract the terminal communicating with the macrocell to the femtocell. In this case, there is a problem that the pilot beacon interferes with the macrocell.
As a countermeasure against such a problem, there has been suggested a technique to stop transmission of a beacon frame if there is no radio station terminal in a wireless area of an access point of a wireless LAN, in order to prevent a negative effect on other systems using the same frequency (for example, see Patent Document 1). In addition, there has been also suggested another technique that a base station adjusts transmission power based on a location of a terminal (for example, see Patent Document 2).